1. Field
The present invention relates to a portable wrist worn device for determining information about the movement of a human body when swimming. The device may also be used in sports other than swimming.
2. Prior Art
There is a requirement to study the motion undertaken by an object or body. In particular this is of interest in the field of sport and athletic endeavours. Athletes need to know how fast and how much distance they have covered during exercise and how this compares with other athletes or past performances. Such a system should be relatively small in size and lightweight, so as not to hinder movement, and be of low cost, low power and high accuracy. It requires the means to process and relay motion data parameters back to the user during or after exercise.
Motion sensing devices are common in the sports of running and cycling and use various techniques such as GPS, mechanical switches, piezoelectric devices and accelerometers to obtain speed and distance information. There is a requirement by athletes engaged in other sports, such as swimming, to obtain speed and distance information.
All swimmers are used to counting laps and aware of the frustration when the total is forgotten or miscounted. There are several devices that allow a swimmer to manually count laps by pressing a button or turning a dial after each lap. U.S. Pat. No. 7,345,958 describes a wrist worn device that shows a swimmers lap count. The swimmer presses a button on the device at the end of each lap to increment the counter. However, this is awkward to use in practice and a swimmer can also forget to press the button to increment the lap. An automatic lap counting system would be much more desirable. A watch was produced by Speedo based on U.S. Pat. No. 5,864,518. This used the conductivity of the water to detect if the watch was immersed. Hence this gave stroke counting information but only for strokes where the arm comes out of the water (back stroke, front crawl and butterfly). However, the operation depended on the electrodes being kept clean and the product was discontinued after a short time. The method described in this document does not rely on conduction through water, has no external electrodes to keep clean and will also work with breaststroke, the most popular of the recreational swim strokes. Although the Speedo device calculated a number of metrics, it did not show a numerical count of a swimmer's laps. The number of laps swam is probably the most useful metric a swimmer can have.
U.S. Pat. No. 5,125,010 describes an automatic lap counting system with radio communication including a wrist worn transmitter and poolside receiver. By employing radio frequency, the lap counting system increments a lap count each time the distance between the receiver and the transmitter becomes smaller than the communication range. Placing a separate receiver on the poolside is inconvenient and undesirable in many public swimming pools and adds significantly to the price of manufacturing the product. There is a need to create a device that automatically counts and relays this information to a swimmer that consists of a single, low cost device worn on the swimmer's body.
MEMS accelerometers are small, sensitive, low power and low cost so are suitable for building into portable devices. US patent 2005/0186542 describes a device worn on the swimmer's back that uses accelerometers to work out lap and stroke data from the change in the acceleration signal on the longitudinal axis of the human body. In practice a device worn on the back has limitations in that it is awkward to position and also requires an additional display or feedback device to be used to relay information back to the swimmer. Additionally this device requires calibration for each individual swimmer which is undesirable to the user. US patent 2008/0018532 describes an invention incorporating accelerometers and a GPS receiver as a data acquisition system for swimmers. GPS receivers have many disadvantages. They are high power, high cost and have difficulty attaining signals indoors. The RF signal associated with GPS is heavily attenuated when immersed in water. Even outdoors they may not be accurate enough to detect changes in laps.
Prior art fails to show a low cost, discrete device that automatically calculates and displays accurate lap counts, stroke counts, speed and distance data for swimmers, for all strokes, that can be worn on the wrist.